Overview
In the communications arena one of the biggest challenges is to overcome crosstalk, noise, and other disturbances that interfere with signals. Whether the signals are transmitted over wires, cable, fiber optics, wireless, or other types of communication the signals suffer from some level of interference.
Interference in the signal may lead to certain limitations of the communication system. For example in wireless systems, such as cellular phones, interference may shorten the distance at which the signal can be reliably received and the clarity of the signal. As another example, in wire systems, such as digital subscriber lines (DSL), interference may shorten the distance at which the signal can be reliably received, i.e., limit loop reach. Interference may also decrease the bit rate of the data being transferred. Providers of telecommunications services recognize the need to monitor the quality of service provided to users of their networks and to identify the causes of problems reported by their customers. This task, however, is complicated significantly by several factors.
Some of these factors include: the large number of network users, the large amount of data collected from the deployed lines, and the presence of competing providers in the same physical line plant. The coexistence of ILECs (Incumbent Local Exchange Carriers) and CLECs (Competitive Local Exchange Carriers) in the same cable binders, brought about by the federally mandated deregulation of local telecommunications markets, implies that services deployed by one carrier may be disturbing the users of another carrier, who has no information about the source of this disturbance.
It is thus highly desirable to sort through the collected data and determine whether a specific line is being disturbed by external interference, such as AM radio stations, or by internal interference, such as another DSL service, and whether that offending service belongs to the same carrier or not. Unfortunately, with today's deployed monitoring technology, carriers are extremely limited in their ability to perform such diagnoses with adequate accuracy and reliability.
The following discussion outlines in detail many of the problems of digital subscriber line (DSL) technology and potential solutions thereto. However, the discussion merely uses DSL as one example of the many communication systems (e.g., wireline, wireless, optical, cable, etc.) in which the present invention may be used. Thus the present invention should not be limited to merely DSL communication systems.
Overview with Respect to DSL
Digital Subscriber Line (DSL) networks provide high speed networking service while preserving the investment made in traditional telephone lines. FIG. 1 shows an exemplary topology of a DSL network. In the exemplary DSL network topology 100 of FIG. 1, various customer premise equipment (CPE) modems 105, 106, 107 are communicatively coupled to a central office switching center 101 via ordinary telephone lines (e.g., lines 120 through 122).
Customer premise equipment 105, 106, 107 is equipment located at the customer's location (e.g., a customer's home or office). In the exemplary network topology 100 of FIG. 1, the customer premise equipment 105, 106, 107 possesses at least one transceiver (e.g., transceiver 108 in CPE 105) that is responsible for: 1) controlling at the CPE the reception of information sent from the service provider; and 2) controlling at the CPE the transmission of information sent to the service provider.
Information that flows in the network 100 toward the customer (e.g., toward the direction of a CPE as seen in FIG. 1) has a “downstream” direction while information that flows in the network 100 away from the customer (e.g., away from a CPE as seen in FIG. 1) has an “upstream” direction. Thus it may be said that a transceiver within a CPE is responsible for controlling at the CPE the transmission of upstream information and the reception of downstream information.
Various DSL service schemes exist. For example, at a high level, DSL services are characterized according to the bandwidth allocated for a customer's upstream and downstream traffic. Services that reserve approximately equal amounts of bandwidth for a customer's upstream and downstream traffic are referred to as “symmetric DSL” while services that reserve approximately unequal amounts of bandwidth for a customer's upstream and downstream traffic are referred to as “asymmetric DSL”.
Symmetric DSL (SDSL), High bit rate DSL (HDSL, HDSL-2) and ISDN DSL (IDSL) are versions of symmetric DSL. Asymmetric DSL (ADSL), Rate Adaptive DSL (RADSL), Very high bit rate DSL (VDSL), and G.Lite are versions of asymmetric DSL. Any of these DSL services (as well as other potential future DSL services that are not listed above) may be referred to as “DSL”.
Note that the central office 101 includes a plurality of DSL Access Multiplexers 102, 103, 104 (DSLAMs). A DSLAM operates as a distributor of DSL services. That is, for example, DSLAM 102 forwards/collects downstream/upstream information sent from/to higher layers of a service provider's network to/from transceivers 108, 109, 110. The service provider's DSL network is controlled by a Network Management Agent (NMA) 118.
An NMA 118 is one or more software routines that monitor the operation of a network (e.g., by collecting various performance monitoring statistics sent from the DSLAMs 102, 103, 104) and controls various aspects of a network (e.g., by enabling or disabling service on a particular line). The NMA 118 shown in FIG. 1 monitors and controls the DSL network 100 by communicating with the DSLAMs through the Element Management Systems 116, 117 (EMSs). The NMA 118, as an example, may be executed as part of a network's Network Management System (NMS). An EMS effectively distributes to the DSLAMs control information sent from the NMA and forwards to the NMA 118 network performance or network status indicia sent from the DSLAMs. More details on a DSL system are provided below.
FIG. 2 shows an exemplary depiction of a receiver 201 within a DSL transceiver 208. That is, for example, transceiver 208 of FIG. 2 may be viewed as corresponding to transceiver 108 of FIG. 1 and line 220 of FIG. 2 may be viewed as corresponding to line 120 of FIG. 1. Recalling that the transceiver 208 is responsible for controlling both the transmission of upstream traffic and the reception of downstream traffic, note that receiver 201 assists the performance of the latter of these two functions.
The receiver 201 includes an equalizer 202 and a symbol detection unit 203 (which may also be referred to as a symbol detector 203). The equalizer 202 adjusts the transfer function of the receive channel such that the frequency components of the received waveform rx(t) 221 that are associated with the signal (i.e., the frequency components of the received waveform rx(t) 221 that are associated with the downstream information sent from the service provider to the transceiver 208) are enhanced with respect to the frequency components of the waveform rx(t) 221 that are not associated with the signal (i.e., the frequency components of the waveform's “noise”). For example, the signal components alone may be amplified, the noise components alone may be suppressed or a combination of both.
The symbol detection unit 203 converts the features of the equalized waveform 222 into digital 1s and 0s according to the modulation scheme employed by the particular type of DSL service being implemented. As a result of the equalizer's activity, the signal-to-noise ratio (SNR) in the receive channel is enhanced and the performance of the symbol detection unit 203 (i.e., its ability to correctly reproduce the digital information sent by the service provider) is improved.
Referring back to FIG. 1, note that the ordinary telephone lines that couple the DSLAMs and the CPEs are tightly packed together in a binder such as binder 114 and binder 115. Because ordinary telephone lines were originally designed for low speed voice/telephony communications, they are typically packed in a binder without shielding. That is, a line is not protected from receiving electromagnetic interference associated with the waveforms that appear on another line; nor are the waveforms on a line prevented from radiating so as to interfere with the waveforms that appear on another line.
The interference described above, commonly referred to as cross-talk, is viewed as noise that may corrupt the operation of the symbol detection unit 203 discussed above with respect to FIG. 2. Cross-talk typically increases as the frequencies of the waveforms on an ordinary telephone line increase.
When the ordinary telephone lines were originally installed to carry voice traffic, cross-talk was insubstantial because of the lower frequencies used to transmit voice traffic. However, as DSL is designed to provide higher speed services (as compared to traditional telephony service) over these ordinary telephone lines, cross-talk from DSL waveforms is much more severe. The more severe cross-talk frequently hampers the successful deployment of a DSL service.